1. Field of the Invention
The present invention relates to sub-stoichiometric dielectric materials. In particular, the present invention relates to sub-stoichiometric dielectric material layers used in optical stacks formed on transparent substrates.
2. Discussion of the Background
Sub-stoichiometric dielectrics are well known in the field of thin films and optical coatings. These sub-stoichiometric materials are typically substances, based on metals, silicon or germanium, that are less than fully reacted with oxygen or nitrogen. Frequently, these materials are optically absorbing in the visible wavelengths while the fully reacted corresponding compounds are often optically non-absorbing in the visible wavelengths.
Sub-stoichiometric materials in thin film form are often included into optical stacks. Compared to stoichiometric dielectric compounds, sub-stoichiometric dielectric materials have a number of desirable properties.
For example, a sub-stoichiometric dielectric generally has a higher index of refraction (“n”) than the corresponding stoichiometric dielectric, and may provide an optical stack with optical properties that are more difficult to reach with all lower index stoichiometric layers. The higher index of the sub-stoichiometric dielectric often allows the sub-stoichiometric layer to be thinner than the corresponding stoichiometric dielectric.
A sub-stoichiometric layer also generally has a higher extinction coefficient (“k”) than the corresponding stoichiometric dielectric. As a result, sub-stoichiometric dielectric materials allow optical stacks to achieve optical properties that cannot be reached with non-absorbing stoichiometric dielectric compounds only.
Thin films of metal sub-oxides and sub-nitrides generally have better properties as chemical barriers than the corresponding stoichiometric metal oxides and nitrides. Barrier layers of metal sub-oxides and sub-nitrides are useful in optical stacks to protect vulnerable metal layers from corrosion. The barrier layers reduce diffusion into the stack of reactive materials such as water or oxygen.
Deposition rates are usually higher for sub-stoichiometric oxide and nitride materials than for stoichiometric materials. As a result, the use of sub-stoichiometric layers typically decreases production costs for manufacturing thin film coatings. This is true for most sputtering and evaporation processes.
For some optical designs, it is desirable to block transmission of UV light. Most sub-stoichiometric materials tend to be more absorbing in the UV wavelengths than the corresponding stoichiometric compounds.
An undesirable property of sub-stoichiometric thin film materials is that they tend to be more chemically reactive than fully oxidized or fully nitrided compounds. Often, a sub-stoichiometric layer in an optical stack will oxidize, particularly if the stack is heated or subjected to water or corrosive chemicals. Oxidation can result in a change in the layer's n and k values, which will change the spectral characteristics of an optical stack.
There is a need to stabilize the properties of optical stacks, particularly optical stacks containing sub-stoichiometric layers. This is particularly true when the stack is heated in an annealing or substrate tempering process.